Inkjet printers eject or “jet” small bursts of liquid ink from extremely fine nozzles to form an image on an image receiving surface, such as an intermediate transfer surface, or a media substrate, such as paper. The benefits of inkjet printing include low printing noise, low cost per printed page, and the ability to print “full color” images. Full color inkjet printers often utilize a print head having four ink reservoirs, one for each color required to generate full color images; namely, cyan, magenta, yellow, and black.
Full color and monochromatic inkjet printers can be divided into two categories based on the properties of the ink ejected from the print head. Aqueous inkjet printers utilize inks formed from a water soluble dye suspended in a polyhydric alcohol. Due to the alcohol base, aqueous inks remain in a liquid phase at room temperature. Therefore, aqueous inks are suitable for printing anytime the user activates the printer. Alternatively, phase change inkjet printers utilize ink that remains in a solid phase at room temperature, often having a waxy consistency. Before the solid ink print head may eject phase change ink, the print head must warm the ink to an operating temperature. As the ink reaches operating temperature it melts, or changes phase, allowing the ink to achieve a liquid consistency suitable for ejection by the print head. The printer maintains at least a portion of the ink at an operating temperature while the printer is activated; however, when the printer is deactivated the ink begins to cool, eventually returning to the solid phase. Before the printer may print an image, the print head must again melt at least a portion of the ink.
Inkjet printers, including phase change or solid ink printers, may include a print head having a comparatively small ink reservoir fluidly connected to a larger main ink reservoir. Each main ink reservoir houses a quantity of liquid ink. The printer contains circuitry that doses portions of the ink in the main reservoir to the print head reservoir. A two-stage reservoir system reduces the mass of the print head, thereby conserving energy and increasing print speed. Additionally, systems utilizing solid ink benefit further because, the small ink reservoir on the print head avoids the necessity of continually applying nominal thermal control to the entire supply of solid ink in the main reservoir for every print job. However, two-stage reservoir systems require the printer to monitor the quantity of ink within the print head reservoir closely, because if the print head completely drains the reservoir during the imaging process, the desired image will not be properly formed. This monitoring is also required for single reservoir systems where the reservoir capacity is small for thermal efficiency.
Various methods exist to monitor the supply of ink in the print head reservoir. One method counts the number of ink drops ejected by the print head nozzle. Ink drop counting works well for theoretical ink volume tracking and provides the printer's electronic controller with a continuously variable ink level signal; however, the system does not provide real-time monitoring of the actual level of ink in the print head and may become increasingly inaccurate with repetitious printing. Ink drop counting is also difficult to track accurately as the amount of ink added to the reservoir cannot always be precisely measured. Consequently, the error differential in the inflow and outflow of ink can accumulate over time. Another method utilizes a pair of electrodes immersed within the print head reservoir. Circuitry monitors the electrical resistance of the ink, and as the ink level changes, so does the resistance measured across the electrodes. Ink resistance monitoring also works well, but only when the printer utilizes an ink with a consistent and measurable conductivity. Optimal control over reservoir volume suffers from an inability to determine existing ink volume in a reservoir accurately and to quantify an ink volume change precisely during both printing and ink replenishment. Identifying an ink volume change is particularly difficult to achieve while simultaneously printing and replenishing. Therefore, further developments in the area of ink level sensing are desirable.